First, the DV request was not timely, so it imposed no cease collection bar.
It could simply be ignored with no response.
Second, even had the DV request been timely, a DV does not impose any requirement to or period for sending validation.
They can choose to simply cease further collection on the debt without any response to a DV.
In the posted scenario, with the debt now paid, the debt collector clearly can no longer pursue collection, so could care less about whether there is a cease collectin bar.
The DV was thus meaningless, and assertion of a violation based on lack of a response is not based on any requirement of the statute.
